Fluorapatite laser material doped with neodymium and manganese



March 3 2 l, 1970 R. MAZELSKY E 3,504,300 -FLUORAPATI TE LASER MATERIAL DOPED WITH NEQDYMIUM AND MANGANESE Filed 001;, s. 1966 2 SheetsSheet l FIG. I

MAG: Nd

HOO

PEmZWFE uuzmowumoz m WAVELENGTH, nm

FIG. 2 INVENTORS ROBERT c. OHLMANN a ROBERT MAZELSKY BY W ATTO WITNESSES Kan/43 g March 31, 1970 R. MAZELSKY .ET AL 3,504,300

FLUORAPATITE LASER MATERIAL DOPED WITH NEODYMIUM AND MANGANESE Filed Oct. 5. 1966 2 Sheets-Sheet 2 o 0 ewwz rtwzmhg wuzwommmoa i WAVELENGTH, n m

FIG.3

ENERGY INPUT (JOULES) United States Patent 3,504,300 FLUORAPATITE LASER MATERIAL DOPED WITH NEODYMIUM AND MANGANESE Robert Mazelsky, Monroeville, and Robert C. Ohlmann, Churchill, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 3, 1966, Ser. No. 583,760 Int. Cl. H01s 3/16 US. Cl. 33194.5 Claims ABSTRACT OF THE DISCLOSURE A crystalline laser material having the formula C35(PO4)3F doped with both neodymium and manganese as partial substitutes for minor portions of the calcium. The material may also be doped with other elements such as selected rare earths, vanadium, titanium, and charge compensators.

This invention relates to a crystal laser material and more particularly it pertains to an alkaline earth halophosphate crystal doped with a rare earth element, with or without other dopants and charge compensators.

The invention described herein was made in the course of or under a contract or subcontract thereunder with the United States Air Force.

This invention is related to and an improvement over copending application Ser. No. 567,918, filed July 26, 1966.

The use of neodymium as a dopant in a number of crystal lattice materials, to obtain laser action having oscillation at optical or near-optical frequencies is well known. Neodymium associated with chromium is used in yttrium aluminum garnet, lanthanum aluminum oxide, and gadolinium oxides. However, these neodymium-doped crystals, such as yttrium aluminum garnet (commonly referred to as YAGzNd) have not been completely satisfactory for use in low-threshold pulse or continuous wave laser operation.

Likewise, other desirable characteristics of a laser crystal such as low threshold power, high gain, and optical transparency to pumping and laser wavelengths, are not all obtained concurrently with high efficiency in those prior known laser materials. Accordingly, there is a critical limitation on the efficiency of optically pumping solid laser as well as on obtaining continuous wave operation.

In accordance with this invention, it has been found that some of the foregoing disadvantages may be overcome by providing a host fluorapatite material which is doped with neodymium or any other rare earth metal having a partially filled 4 electronic shell as a partial substitute for calcium. Such host material when doped with neodymium has a high gain and a lower threshold whichexceeds the best existing materials.

Accordingly, it is a general object of this invention to provide a crystal laser material having low pulse threshold and which is suitable for continuous wave laser operation.

It is another object of this invention to provide a crystal laser material having a high gain and higher efli'ciencies than prior known host materials.

Finally, it is an object of this invention to satisfy the foregoing objects and desiderata in a simple and expedient manner.

Briefly, the present invention involves a crystalline laser material which is composed of calcium fluorapatite (calcium fluorophosphate) and which is doped with neodym- "ice ium as a partial substitute for the calcium therein, hereinafter referred to as FAPzNd. Further, as will be disclosed hereinafter, other elements such as selected rare earths, manganese, and charge compensators may be substituted with PAP.

For a better understanding of the invention, reference is made to the drawings, in which:

FIGURE 1 is a perspective view, partly broken away, of an apparatus using a laser rod;

FIG. 2 is a graph showing the fluorescence intensity versus wavelength of the emission of neodymium in fluorapatite and in yttrium aluminum garnet;

FIG. 3 is a graph of the fluorescence intensity versus wavelength of the emission of manganese ions and of neodymium absorption in fluorapatite at room temperature; and

FIG. 4 is a graph of energy output versus energy input of fluorapatite doped with varying amounts of neodymium and yttrium aluminum garnet doped with neodymium.

There is illustrated a laser in FIG. 1, wherein a rodshaped crystal 10 is disposed within a cylindrical light filter 12 which in turn is mounted within a cylindrical reflector 14. An elongated power supply lamp 16 is mounted within the cylinder 14, the lamp 16 being substantially parallel with the crystal 10. Opposite ends of the cylinder 14 are closed by end panels 18 and 20'.

The crystal 10, the tube 12, and the lamp 16 are substantially coextensive with the length of cylinder 14 and extend between the opposite end panels 18 and 20. The lamp 16 is provided with lead wires 22 extending from opposite ends thereof to a suitable power supply which is not shown. As an example, the lamp 16 is a Xenon-filled flash lamp of mm. arc length, and 5 mm. inside diameter. The light filter 12 may be a glass tube for filtering out selective wave lengths of the pump light such as ultraviolet waves which are detrimental to laser action. Examples of suitable glasses are quartz and -borosilicate glass sold under the trademark of Pyrex. The inner surfaces of the cylinder 14 as well as of the end panels 18 and 20 have a reflective aluminized surface 24 such that light emanating from the lamp 16 is reflected to the crystal 10 in a manner shown by the broken lines 26. It is preferred that all light entering the crystal 10 pass through the filter 12.

Each end panel 18 and 20 is provided with an opening 28 which is aligned with the end of the crystal 10. Reflective means are associated with each end of the rod crystal 10, for instance a pair of mirrors 30 and 32 are disposed in alignment with the axis of the crystal 10, the mirror 30 being partially reflective and partially transmittive and the mirror 32 being totally reflective of laser beams 34 projected from the ends of the crystal 10 to the mirrors. As a result of the partial reflectivity of the mirror 30, some of the beams 34 pass through the mirror as shown at 36. Alternately the ends of rod crystal 10 can be approximately silvered to effect the same result.

In accordance with the present invention the rodshaped crystal 10 is a single crystal of fluorapatite Ca (PO F (FAP) which is doped with neodymium. A technique for making the single :crystal is that commonly called the Czochralski method and involves the wetting of a seed crystal by the molten FAP salt and slowly crystallizing the salt on the seed as the seed crystal is withdrawn. For fluorapatite, the molten liquid is held in a container of iridium or other non-reactive noble metal or alloy with melting points in excess of 1750 C. The seed crystal, while being pulled, is rotated at from 10 to r.p.m. and withdrawn from the melt at rates between 1 and 8 mm. per hour. However, pull and rotation rates faster and slower than those indicated may be suitable for growing laser crystals. After a crystal is prepared, it is then ground and polished to the desired dimension to J provide a cylindrical rod with its ends polished to optical quality, the ends being precisely parallel to each other, and perpendicular to the rod axis, as is well known.

The laser crystal in accordance with the invention comprises crystalline calcium fluorophosphate,

which is the mineral fluorapatite (FAP). Fluorapatite as a host material may be converted to a material capable of the production of laser action by the addition of a dopant to the material, such dopants consisting of ions of the trivalent rare earth 4 type elements including particularly europium, terbium, holmium, thulium, erbium, neodymium, praseodymium, and ytterbium. Neodymium has proven to be outstanding. The addition of neodymium or any other dopant from the group of rare earth elements is made as a substitute for a minor portion of the calcium in fluorapatite.

A crystal of the halophosphate of the apatite structure may be prepared in any feasible manner at an elevated temperature as is indicated by one of the formulas:

Reaction (2) is preferred. Intermediate reactions may occur, but the foregoing formulas represent the basic ultimate reactions.

The dopant ion, Nd, is incorporated by replacement of calcium ions in either the phosphate or the fluoride reactants. Thus, Nd O or NdF may be substituted for a part of the CaF or CaCO or combinations thereof in the above reactions. Other neodymium materials such as Nd2(C204)3, Nd2(SO4)3, and Nd2(PO4)3 may be used to produce a useful laser crystal.

However, the amount of neodymium need not be added in stoichiometric proportions substituting for calcium so long as the resultant crystal is essentially the apatite structure.

EXAMPLE I A typical Weight ratio for materials used to prepare a crystal of FAPzNd is:

caHPo, 44.348

El it??? CaCO 15.000

These finely divided materials are admixed and melted together at about 1700 C. until completely reacted. A single crystal is pulled by the Czochralski method. The above melt composition contains a nominal neodymium doping of about 2 atom percent but the resultant crystal contains a lower concentration of neodymium (Nd in the typical hexagonal crystal structure of apatite.

In a similar manner the oxide or carbonate of europium, terbium, holmium, thulium, erbium, praseodymium, and ytterbium can be substituted for a part of or all of the neodymium oxide in Example I.

In general the neodymium doped fluorapatite has the following nominal formula:

Where x has a value of from 0.001 to 0.2. For laser use, the optimum value of x is 0.05 to 0.1. For example, where x:.06, the formula is Ca N ,05(PO F.

The fluorescence spectra of neodymium in crystals of PAP and YAG are illustrated in FIG. 2. The FAPzNd 0.4% spectrum is characteristic of the spectra for the electric field perpendicular to the c-axis of the crystal (E l c). The spectra for E l I c is very similar except that the principal emission line is even more intense (by about 2.5 times) compared to the other lines. The fluorescence spectra of Nd show that in PAP the emission is somewhat unique compared to Nd emission in YAG in that a much larger fraction of the emission is found in a single line. The FAPzNd line is located at 1.0629,u. and is about 6 angstroms wide for E c. The internal emission efliciency at 1.0629 for FAPzNd 0.4% for E{ [c is 2.6:03 times greater than that of YAGzNd 1.5% at 1.0648 The YAGzNd line is 6.5 angstroms wide and is less intense. Thus, the spectral eflflciency at room temperature is about 2.6 times greater for Nd in PAP than in YAG. The stimulated emission gain for a given rate of absorbing energy will be correspondingly greater for FAP than YAG. The Nd absorption and excitation spectra are similar for the two materials although the absorption in PAP is somewhat greater than in YAG. For the same cavity losses the continuous wave threshold of a FAPzNd laser rod should be a maximum of one-third that of the same size YAGzNd rod.

In order to maintain electroneutrality without formation of lattice vacancies an additional element or elements may be added to compensate for the valence difference between calcium and neodymium. Thus, the substitution of a trivalent rare earth ion in a divalent calcium site requires charge compensation by the addition of either a monovalent cation or a divalent anion in the calcium or fluoride sites, respectively. Suitable monovalent ions for every neodymium substitution in a calcium site include at least one of the alkali metals such as lithium, sodium, potassium, and rubidium. The ionic radii of the other metals in the alkali group, namely cesium and francium, are believed to be too large for substitution in the calcium site. Lithium, sodium, potassium, and rubidium is added as a charge compensator for calcium in an amount up to the mole proportion of the neodymium present. Because its ionic size is substantially equal to that of calcium, sodium is the preferred element for substitution to obtain the required charge compensation. The nominal formula for the laser material which is doped with neodymium and charge compensated by monovalent ions is as follows:

x 52x x( 4) 3F where M is lithium, sodium, potassium, and/ or rubidium, and x as a value of from 0.001 to 0.2.

The reaction to produce alkali metal compensated FAPzNd is carried out as in Example I. For a sodiumcompensated typical charge for the initial melt, the following is exemplary:

Gms. caco, 15.000 CaHPO 47.760 Cal-" 4.501 Nd O 1.939 Na CO When melted and crystallized as in Example I this gives a nominal composition of:

fluorine ions is:

5x x( O4)3F1-XOX where x varies from 0.001 to 0.2; the optimum value of x being between 0.05 and 0.1. Oxygen may be conveniently added as Nd203.

A specific example of a crystal of fluorapatite doped with neodymium and charge compensated with oxygen is:

Compensation to maintain electroneutrality in the crystal may occur in other ways, such as by the presence of calcium vacancies or interstitial fluoride ions, which is self compensation. Self compensation of the crystal, however, is undesirable as an alternative to charge compensation where a mono-valent ion such as sodium is added to the calcium site or where a divalent anion such as oxygen is added as a partial substitute for the fluorine ion to keep the ionic charges in the crystal in balance. It has been found that where such cations or anions are not added as charged compensators and the crystal is permitted to self compensate by induced vacancies in the lattice, the crystal ultimately develops a color center which reduces the optical properties and overall efliciency of the crystal.

Though calcium is the preferred alkaline earth metal in the PAP, it is understood that other divalent metals including magnesium, strontium, and barium may be used to replace a part or all of the calcium. Table I lists these alkaline earth metals together with their atomic size in angstroms.

TABLEI Goldschmidts Alkaline earth metal: ionic radii, angstroms Magnesium 0.78 Calcium 1.06 Strontium 1.27 Barium 1.43

From the table, it is evident that magnesium is smaller that calcium by 20% for which reason it is difficult to completely substitute magnesium in the calcium site in apatite while retaining all its properties. However, the alkaline earth metals either individually or in combination may be substituted for calcium in varying amounts and still retain the apatite structure. For example Sr (PO F has the apatite crystal structure and is suitable as a laser host material.

In accordance with this invention, the efficiency of the laser may be improved by the simultaneous or double doping of fiuorapatite with both manganese and neodymium. As shown in FIG. 3, the fluorescence spectrum of manganese overlaps the absorption spectrum of neodymium. Manganese absorbs energy in spectra regions where neodymium does not absorb even though there are some regions where the absorption spectra for both metals are identical or overlapping. The energy absorbed by manganese may be subsequently transferred to the neodymium ions due to the overlap shown in FIG. 3 and thereby enhance the output energy of these ions.

Manganese can be substituted directly in the calcium of manganese replacing an equimolar quantity of calcium. Other divalent transition metal ions may also be used to sensitize neodymium doped PAP, and particularly vanadium and titanium, both of which like manganese have a valence of 2 and any one or any two or all three of manganese, vanadium, and titanium may be directly substituted for a part of the calcium.

The use of manganese with neodymium to produce FAPzMmNd may produce a significant improvement in the efficiency over FAPINd both in continuous and pulse lasers. Manganese introduces elfective pumping bands at wavelengths not absorbed directly by the neodymium. Fluorapatite is an extremely attractive host for manganese sensitization because a high concentration of manganese may be substituted for calcium. Moreover, fiuorapatite which is doped with neodymium and manganese may also be charge compensated by both cations and anions. The general formula for such material is 5-xyw x y w PO 4) 3 1-2 2 where x has a value of from 0.001 to 0.2, y has a value of from 0.0 up to the solubility limit of manganese, 2 has a value of from 0.0 up to the limit of lattice stability, w has a value of from 0.0 to that of x, and M is at least one of the alkali metals selected from the group consisting of lithium, sodium, potassium, and rubidium. The charge neutrality must be maintained, which requires that x=w+z.

The following example is further illustrative of the present invention:

EXAMPLE II The performance of laser crystals of fiuorapatite doped with varying amounts of neodymium (FAP:Nd) was compared with that of the presently best known crystal material, i.e. yttrium aluminum garnet doped with neodymium (YAG:Nd). One YAG crystal doped with 1.5 mole percent neodymium having a rod size of 0.25 inch diameter and a length of 1.48 inch was prepared and tested. Likewise, three crystals of PAP doped with neodymium were prepared and tested. One FAP crystal contained approximately 1.3 mole percent neodymium and had a diameter of 0.25 inch and a length of 2.0 inch. The second FAP crystal contained approximately 1.1 mole percent neodymium and had a diameter of 0.178 inch and a length of 1.75 inch. The third PAP crystal contained approximately 1.4 mole percent neodymium, had a diameter of 0.24 inch and a length of 1.73 inch. The parameters and test results of the YAG:Nd crystal and of the three FAP: Nd crystals are listed in Table II.

TABLE II.LASER TESTS CONDUCTED WITH ONE YAG:Nd ROD AND THREE FAPzNd RODS Gain Coef. per Maximum Lowest threshwhere y has a value of from 0.0 up to the solubility limit of manganese, and x has a value of from 0.001 to 0.2.

Accordingly, crystals of neodymium doped fiuorapatite may be sensitized by incorporating up to 10 mole percent The gain coeflicient per pumped energy shows a marked improvement for the FAPzNd crystals over that of the YAG:Nd crystal. Finally, the efficiency of the FAPzNd crystals exceeds that of the YAG:Nd crystal and the results of this test are shown in FIG. 4 where the energy input in joules is plotted against the energy output. In FIG. 4, the maximum slope efliciency of the YAG:Nd crystal is 1.65 percent which is substantially less than the latest prepared FAP:Nd crystal having an efficiency slope of 2.4 percent.

Accordingly, the fiuorapatite doped with neodymium is an improvement over prior known laser materials such as yttrium aluminum garnet for decreased laser threshold power, increased gain, and improved efliciency for use in lasers which oscillate at about 1.06,u wave length. Moreover, fiuorapatite doped with neodymium has 3 to 5 times higher gain than yttrium aluminum garnet doped with neodymium for the same energy absorbed. Thus, it is easier to stimulate laser action in the fiuorapatite than in the garnet.

It is understood that the above specification and drawings are merely exemplary and not in limitation of the invention.

What is claimed is:

1. A crystal suitable for laser application consisting essentially of a host fiuorapatite active laser material having the formula Ca (PO F, the material being doped with neondymium as a substitute for a minor portion of the calcium therein, and the material also being doped With at least one of a metal selected from a group consisting of manganese, vanadium, and titanium as a substitute for a minor portion of the calcium up to the solubility limit thereof.

2. The crystal of claim 1 wherein oxygen ions are partially substituted for the fluorine ions, and having the formula Ca Nd Mn (PO F O where x has a value of from 0.001 to 0.2, and y has a value of up to the solubility limit of manganese.

3. The crystal of claim 2 having the formula where w has a value of up to that of x, and M is at least one of the alkali metals selected from the group consisting of lithium, sodium, potassium, and rubidium, and 2 has a value of up to the limit of lattice stability, and x=w+z.

4. A laser comprising a light source, a cylindrical rod consisting essentially of an active laser material having the formula: Ca Nd Mn (PO F O, where x has a value of from 0.001 to 0.2, and y has a value of from 0.0 to the solubility limit of manganese, the rod having flat parallel ends, and reflective means disposed parallel to each end.

5. A laser comprising a light source, a cylindrical rod consisting essentially of an active laser material having the formula: Ca Nd Mn M (PO F O where x has a value of from 0.001 to 0.2, y has a value of up to x, where z has a value of up to the limit of lattice stability, where w has a value of up to that of x, and M is at least one of the alkali metals selected from the group consisting of lithium, sodium, potassium, and rubidium, the hod having flat parallel ends, and reflective means disposed parallel to each end.

References Cited UNITED STATES PATENTS 3,252,103 5/1966 Geusic et al. 33194.5 X 3,289,100 11/1966 Ballman et al. 33194.5

ROY LAKE, Primary Examiner SIEGFRIED H. GRIMM, Assistant Examiner US. Cl. X.R. 

